1CKSUM(1P) POSIX Programmer's Manual CKSUM(1P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 cksum - write file checksums and sizes
13
15 cksum [file ...]
16
18 The cksum utility shall calculate and write to standard output a cyclic
19 redundancy check (CRC) for each input file, and also write to standard
20 output the number of octets in each file. The CRC used is based on the
21 polynomial used for CRC error checking in the ISO/IEC 8802-3:1996 stan‐
22 dard (Ethernet).
23 The encoding for the CRC checksum is defined by the generating polyno‐
25 mial:
26 G(x)=x**32+x**26+x**23+x**22+x**16+x**12+x**11+x**10+x**8+x**7+x**5+x**4+x**2+x+1
29 Mathematically, the CRC value corresponding to a given file shall be
31 defined by the following procedure:
32 1. The n bits to be evaluated are considered to be the coefficients of
34 a mod 2 polynomial M( x) of degree n-1. These n bits are the bits
35 from the file, with the most significant bit being the most signif‐
36 icant bit of the first octet of the file and the last bit being the
37 least significant bit of the last octet, padded with zero bits (if
38 necessary) to achieve an integral number of octets, followed by one
39 or more octets representing the length of the file as a binary
40 value, least significant octet first. The smallest number of octets
41 capable of representing this integer shall be used.
42 2. M( x) is multiplied by x**32 (that is, shifted left 32 bits) and
44 divided by G( x) using mod 2 division, producing a remainder R( x)
45 of degree <= 31.
46 3. The coefficients of R( x) are considered to be a 32-bit sequence.
48 4. The bit sequence is complemented and the result is the CRC.
50
52 None.
53
55 The following operand shall be supported:
56 file A pathname of a file to be checked. If no file operands are
58 specified, the standard input shall be used.
59
62 The standard input shall be used only if no file operands are speci‐
63 fied. See the INPUT FILES section.
64
66 The input files can be any file type.
67
69 The following environment variables shall affect the execution of
70 cksum:
71 LANG Provide a default value for the internationalization variables
73 that are unset or null. (See the Base Definitions volume of
74 IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
75 ables for the precedence of internationalization variables used
76 to determine the values of locale categories.)
77 LC_ALL If set to a non-empty string value, override the values of all
79 the other internationalization variables.
80 LC_CTYPE
82 Determine the locale for the interpretation of sequences of
83 bytes of text data as characters (for example, single-byte as
84 opposed to multi-byte characters in arguments).
85 LC_MESSAGES
87 Determine the locale that should be used to affect the format
88 and contents of diagnostic messages written to standard error.
89 NLSPATH
91 Determine the location of message catalogs for the processing of
92 LC_MESSAGES .
93
96 Default.
97
99 For each file processed successfully, the cksum utility shall write in
100 the following format:
101 "%u %d %s\n", <checksum>, <# of octets>, <pathname>
104 If no file operand was specified, the pathname and its leading <space>
106 shall be omitted.
107
109 The standard error shall be used only for diagnostic messages.
110
112 None.
113
115 None.
116
118 The following exit values shall be returned:
119 0 All files were processed successfully.
121 >0 An error occurred.
123
126 Default.
127 The following sections are informative.
129
131 The cksum utility is typically used to quickly compare a suspect file
132 against a trusted version of the same, such as to ensure that files
133 transmitted over noisy media arrive intact. However, this comparison
134 cannot be considered cryptographically secure. The chances of a damaged
135 file producing the same CRC as the original are small; deliberate
136 deception is difficult, but probably not impossible.
137 Although input files to cksum can be any type, the results need not be
139 what would be expected on character special device files or on file
140 types not described by the System Interfaces volume of
141 IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001 does
142 not specify the block size used when doing input, checksums of charac‐
143 ter special files need not process all of the data in those files.
144 The algorithm is expressed in terms of a bitstream divided into octets.
146 If a file is transmitted between two systems and undergoes any data
147 transformation (such as changing little-endian byte ordering to big-
148 endian), identical CRC values cannot be expected. Implementations per‐
149 forming such transformations may extend cksum to handle such situa‐
150 tions.
151
153 None.
154
156 The following C-language program can be used as a model to describe the
157 algorithm. It assumes that a char is one octet. It also assumes that
158 the entire file is available for one pass through the function. This
159 was done for simplicity in demonstrating the algorithm, rather than as
160 an implementation model.
161 static unsigned long crctab[] = {
164 0x00000000,
165 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
166 0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
167 0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
168 0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
169 0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
170 0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
171 0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
172 0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
173 0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
174 0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
175 0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
176 0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
177 0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
178 0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
179 0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
180 0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
181 0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
182 0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
183 0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
184 0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
185 0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
186 0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
187 0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
188 0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
189 0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
190 0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
191 0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
192 0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
193 0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
194 0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
195 0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
196 0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
197 0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
198 0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
199 0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
200 0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
201 0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
202 0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
203 0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
204 0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
205 0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
206 0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
207 0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
208 0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
209 0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
210 0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
211 0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
212 0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
213 0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
214 0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
215 0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
216 };
217 unsigned long memcrc(const unsigned char *b, size_t n)
220 {
221 /* Input arguments:
222 * const char* b == byte sequence to checksum
223 * size_t n == length of sequence
224 */
225 register unsigned i, c, s = 0;
228 for (i = n; i > 0; --i) {
231 c = (unsigned)(*b++);
232 s = (s << 8) ^ crctab[(s >> 24) ^ c];
233 }
234 /* Extend with the length of the string. */
237 while (n != 0) {
238 c = n & 0377;
239 n >>= 8;
240 s = (s << 8) ^ crctab[(s >> 24) ^ c];
241 }
242 return ~s;
245 }
246 The historical practice of writing the number of "blocks" has been
248 changed to writing the number of octets, since the latter is not only
249 more useful, but also since historical implementations have not been
250 consistent in defining what a "block" meant. Octets are used instead
251 of bytes because bytes can differ in size between systems.
252 The algorithm used was selected to increase the operational robustness
254 of cksum. Neither the System V nor BSD sum algorithm was selected.
255 Since each of these was different and each was the default behavior on
256 those systems, no realistic compromise was available if either were
257 selected-some set of historical applications would break. Therefore,
258 the name was changed to cksum. Although the historical sum commands
259 will probably continue to be provided for many years, programs designed
260 for portability across systems should use the new name.
261 The algorithm selected is based on that used by the ISO/IEC 8802-3:1996
263 standard (Ethernet) for the frame check sequence field. The algorithm
264 used does not match the technical definition of a checksum; the term is
265 used for historical reasons. The length of the file is included in the
266 CRC calculation because this parallels inclusion of a length field by
267 Ethernet in its CRC, but also because it guards against inadvertent
268 collisions between files that begin with different series of zero
269 octets. The chance that two different files produce identical CRCs is
270 much greater when their lengths are not considered. Keeping the length
271 and the checksum of the file itself separate would yield a slightly
272 more robust algorithm, but historical usage has always been that a sin‐
273 gle number (the checksum as printed) represents the signature of the
274 file. It was decided that historical usage was the more important con‐
275 sideration.
276 Early proposals contained modifications to the Ethernet algorithm that
278 involved extracting table values whenever an intermediate result became
279 zero. This was demonstrated to be less robust than the current method
280 and mathematically difficult to describe or justify.
281 The calculation used is identical to that given in pseudo-code in the
283 referenced Sarwate article. The pseudo-code rendition is:
284 X <- 0; Y <- 0;
287 for i <- m -1 step -1 until 0 do
288 begin
289 T <- X(1) ^ A[i];
290 X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
291 comment: f[T] and f'[T] denote the T-th words in the
292 table f and f' ;
293 X <- X ^ f[T]; Y <- Y ^ f'[T];
294 end
295 The pseudo-code is reproduced exactly as given; however, note that in
297 the case of cksum, A[i] represents a byte of the file, the words X and
298 Y are treated as a single 32-bit value, and the tables f and f' are a
299 single table containing 32-bit values.
300 The referenced Sarwate article also discusses generating the table.
302
304 None.
305
307 None.
308
310 Portions of this text are reprinted and reproduced in electronic form
311 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
312 -- Portable Operating System Interface (POSIX), The Open Group Base
313 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
314 Electrical and Electronics Engineers, Inc and The Open Group. In the
315 event of any discrepancy between this version and the original IEEE and
316 The Open Group Standard, the original IEEE and The Open Group Standard
317 is the referee document. The original Standard can be obtained online
318 at http://www.opengroup.org/unix/online.html .
319IEEE/The Open Group 2003 CKSUM(1P)